1. Field of the Invention
The present invention relates to a method and an apparatus for pulse width modulating a display device such as a spatial light modulator.
2. Related Art
A spatial light modulator (SLM) is device which imparts information onto a light beam. For example, SLMs include liquid crystal devices (LCD—reflective and transmissive) and micro-electronic mirror systems (MEMS). SLMs are useful as part of display devices. One known type of display device utilizing an SLM is an LCD having a liquid crystal (LC) material which is driven by electronics located under each pixel. There are many known pixel architectures for these devices, each of which utilizes different structures and techniques to drive the LC material. For example, an analog pixel architecture might represent the color value of the pixel with a voltage that is stored on a capacitor under the pixel. This voltage can then directly drive the LC material to produce different levels of intensity on the optical output.
Digital pixel architectures store the pixel value as a digital value in a memory device (e.g., DRAM, SRAM, etc). In this case, the digital information is generally converted to an analog form to drive the LC material. One common approach to such conversion is pulse-width modulation (PWM). For example, PWM is one technique for generating gray scale in an SLM device. In this approach, the LC material is driven by a digital waveform whose active time is a function of the desired gray scale value. With reference to FIG. 1, an example pulse width modulation waveform includes two refresh cycles. Each refresh cycle has a refresh time tr and each pulse in the refresh cycle has an active time ton.
The active time of the PWM waveform, ton, is a function, fpwm, of the current pixel value, p, where p is an integer value between 0 and 2n−1, n is the number of bits in a color component (typically 8 for many computer systems), ton is a number between 0 and tr, and tr is a refresh time (generally constant). For example, if fpwm is linear, then ton is given by the expression:
                              t          on                =                                            f              pwm                        ⁡                          (              p              )                                =                                    p                              2                n                                      ⁢                          t              r                                                          Eq        .                                  ⁢                  (          1          )                    The refresh time depends on the response time of the material along with the update rate of the content that the device displays. Ideally, the refresh time should be shorter than that of the content and the minimum active time should be larger than the response time of the LC material.
The active time ton is also time varying as the pixel value p may change over time. It is often desirable to use a non-linear function for fpwm to match this function with other non-linear aspects of the system. Also, to provide for DC-balanced drive of the LC material, the PWM signal may be inverted every other frame before being provided to the LC material (i.e., the active time is active-high during even refresh cycles and active-low during odd refresh cycles).
In some systems, the SLM multiplexes several colors sequentially in time. This type of multiplexing is a common low-cost approach to generating full color from the inherently gray-scale SLM devices. With reference to FIG. 2, an example pulse width modulation waveform includes one refresh cycle for red (R), one refresh cycle for green (G), and one refresh cycle for blue (B). The entire video frame includes three refresh cycles, namely one each for R, G, and B. During each refresh cycle, the SLM displays either red, green, or blue data. Multiplexing several colors onto a single SLM generally requires additional color management hardware and/or software in the display system that is responsible for ensuring that the appropriate color illuminates the SLM at the appropriate point in time. In some systems, two colors are multiplexed onto a first SLM device and the third color is continuously provided to a second SLM device. Also, some colors may be repeated within a particular video frame (e.g. R-R-G-B or R-G-R-B).